Autogenous shrinkage

Shrinkage Development in High
Performance Concrete
Ammar Yahia, P.Eng., Ph.D.,
Outlines
1. Introduction
2. High-performance concrete
3. Autogenous shrinkage
5
4. Autogenous shrinkage stress
5. Autogenous Shrinkage measurements
6. Perspectives
Questions & discussions
6
1
2
4
3
Introduction
• The middle east is currently one of the
greatest construction location in the word
• Trend in concrete technology towards low
w/cm - HPC.
• Increased tendency to undergo early-age
cracking - may or may not compromise the
(higher) f’c, it likely does compromise their
long-term performance.
Structural & Economical benefits
• HPC become a crucial element of the viability
of tall building in the region
• High strength to reduce the size of sections
• Stiffness provided by high modulus –
limiting movements
• Durability of HPC - achieving service life
requirements
Durability requirements
Severe environment
Confederation Bridge - Canada
w/cm = 0.27, 90 MPa, Ternary binder
100 years service life!!
Ternary binder, fly ash + silica fume
Durability improvement
• Durability improvement of HPC is achieved by
eliminating the transport mechanisms into the concrete
• Modification of the mixture proportions
• Higher cement content
• Supplementary cementitious materials (GGBS, SF, FA)
• Lower w/cm and extensive use of superplasticizer
Consequences
• Due to the mixture proportion changes – HPC develop
higher early-age shrinkage – cracking that can reduce
the service life of concrete structures
Moisture-related shrinkage
• Plastic shrinkage
• Drying shrinkage – loss of moisture
• Carbonation shrinkage – carbonation effect
• Chemical shrinkage
• Autogenous shrinkage
Self-desiccation shrinkage
Le Chatelier's Experiment
• When cement paste hardening:
underwater
In air
Level
water
V
Before
After
Apparent volume shrinks
Before
After
Apparent volume swells
• Cement hydration creates a certain porosity
Jensen and Hansen's Model
W/C = 0.42
(1,1)
(0,1)
Pores
About 8%
Relative volume
Capillary
water
(0,0)
l
e
g
ter
a
W
Cement gel
Anhydrous
cement
α
Degree of hydration
(1,0)
Jensen and Hansen's Model
W/C = 0.42
External water
(1,1)
(0,1)
Relative volume
Capillary
water
(0,0)
l
e
g
ter
a
W
Cement gel
Anhydrous
cement
α
Degree of hydration
(1,0)
Normal concrete vs. HPC
• The shrinkage behavior of HPC is quite different from
the shrinkage of normal concrete (NC)
• NC: drying shrinkage is the main component
• HPC: autogensous shrinkage is the main
component
• This difference can be explained in the size of the
pore and capillary network
HPC critical characteristics
• W/C equal or smaller than 0.36
• Missing water to fully hydrate cement particles
• Elastic modulus depends on elastic modulus of
the coarse aggregate
• Very sensitive to autogenous shrinkage
Autogenous shrinkage
• Autogenous shrinkage = macro-volume reduction
observed after the initial set. It is induced by the selfdesiccation that occurs during hydration under sealed
or partially saturated conditions
• As the cementitious materials hydrates under sealed
conditions, empty porosity is created within the ‘set’
microstructure – hydration product occupy less
volume
Autogenous shrinkage
• The creation of empty capillary pore spaces has two
major effects on the evolving cement paste system:
• The chemical shrinkage results in a reduction in the
system internal relative humidity (quantified by
Gauss and Tucker in 1940) – 70% for w/c < 0.30
• Based on the Kelvin-Laplace equation, this reduced
RH will induce pressure σcappilary in the pore water
Autogenous shrinkage
• The magnitude of these stresses is influenced by both
the surface tension of the pore solution and the
meniscus radius of the largest water-filled pore water
within the microstructure
Kelvin-Laplace Equation
⎛ RH ⎞
Ln⎜
⎟ RT
2γ
100 ⎠
⎝
σ cap = = −
r
Vm
RH = relative humidity expressed as percentage
γ = surface tension
r = radius of the largest water-filled pore,
R = universal gas constant (8.314 J/(mol.K))
T = absolute temperature
Vm = molar volume of pore solution
What will happen if RH decreases from 95% to 70%?
Kelvin-Laplace Equation
⎛ RH ⎞
Ln⎜
⎟ RT
2γ
100 ⎠
⎝
σ cap = = −
r
Vm
Sσ cap 1 1
ε=
( − )
3 K Ks
S = degree of saturation
K = bulk modulus of elasticity (porous materials)
Ks = bulk modulus of solid framework within
porous materials
Autogenous shrinkage
• The second effect of the creation of empty capillary
pores is a change in the hydration kinetics of the cement
paste
• cement hydration proceed by dissolution/precipitation
mechanisms
• The empty pore space created is no longer available
to be filled with hydration product – slow down
Measurements
Water Curing
(Aticin, 1998)
Effect of Binder Type
2.0
Concrete prisms
Ternary Binder 2
1.8
Humidity loss (%)
1.6
w/c = 0.35
1.4
1.2
Ternary Binder 1
1.0
0.8
0.6
Ternary Binder 3
0.4
0.2
0.0
0
1
2
3
4
5
6
7
8
Time (hours)
Yahia and Khayat (not published)
Effect of Binder Type
Time (hours)
0
1
2
3
4
5
6
7
8
9
10 11
12
13 14
15
Concrete prisms
200
Ternary binder 2
Micro-Strains
100
Ternary binder 3
w/cm = 0.35
0
-100
-200
Ternary Binder 1
-300
-400
Yahia and Khayat (not published)
External Water Curing
• Does not penetrate very far into low water/binder
concretes - HPC
• It should be done very early to reduce autogenous
shrinkage
• But maximum efficiency is 1 to 2 days when capillary
pores are still interconnected. Otherwise, external
curing is no more efficient!!
The issue
Shrinkage is a weakness of concrete when it induce more
or less severe cracking, because cracking
can reduce concrete service life and durability
Any micro- and/or macro-cracks network that is
developing in concrete due to shrinkage offers pathways
to aggressive agent - will attack steel reinforcing bars
and result in a weakening of the structure
Perspectives
• Use internal water sources to reduce self-desiccation
- Saturated lightweight aggregates (20% - 25%)
(Teaming-up with colleagues from Canada)
- Chemical reducing admixtures
- Super-absorbent polymers (SAP)